I just had to buy two new rears for my factory 20's and it took me a LONG time to track down a set of new Michelin Latitude Tour HP to match my 85% remaining fronts.
BUT in the end, I was able to, and I did not have to buy 4 new tires and have 2 fronts in near perfect condition to sell to someone else, or throw out.
So check the usual suspects for used tires and then look at new, just to make sure you're not getting hosed on the used tires prices.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S